Chemotherapy is still one of the most important anti-tumor strategies, including surgery, radiation therapy, and targeted therapy methods. Although there are many types of highly efficient cytotoxins, the small difference between cancer cells and normal cells limits the extensive use of these anti-tumor compounds in the clinic due to the toxicity and side effects. Besides, the specificity of anti-tumor monoclonal antibodies against tumor cell surface antigens makes antibody drugs the first-line anti-tumor therapy drugs. However, the efficacy is often unsatisfactory when the antibody is used alone as the anti-tumor drug.
Antibody drug conjugate (ADC) means connecting a monoclonal antibody or antibody fragment with a bioactive cytotoxin via a stable chemical linker compound, taking full advantage of the binding specificity of the antibody to normal and tumor cell surface antigens and the high efficacy of cytotoxins, meanwhile avoiding low efficacy of the former and excessive side effects of the latter. This means that, compared with conventional traditional chemotherapy drugs, the antibody drug conjugate is capable of binding precisely to the tumor cells and reducing the impact on the normal cells (Mullard A, (2013) Nature Reviews Drug Discovery, 12:329-332; DiJoseph J F, Armellino D C, (2004) Blood, 103:1807-1814).
Early ADC drugs primarily used murine monoclonal antibodies, some of which had difficulties reaching the target as a result of the human immune response. Secondly, effector molecules, including doxorubicin used in the early stage, exhibited lower biological activity, which limited the efficacy of the first generation of antibody drug conjugates. In addition, the source of antibodies, the linking mode and number of linkers have not yet been optimized.
In 2000, the first antibody drug conjugate Mylotarg® (gemtuzumab ozogamicin, Wyeth Pharmaceuticals) was approved by the US Food and Drug Administration (FDA) for the treatment of acute myeloid leukemia (Drugs of the Future (2000) 25(7):686; U.S. Pat. Nos. 4,970,198; 5,079,233; 5,585,089; 5,606,040; 5,693,762; 5,739,116; 5,767,285; 5,773,001). Mylotarg® is a humanized CD33 antibody-calicheamicin conjugate, which was withdrawn by Pfizer itself in 2010 because of the limited efficacy and high toxicity.
August 2011, Adcetris® (brentuximab vedotin, Seattle Genetics Inc.) was approved through the US FDA Fast Track for the treatment of Hodgkin lymphoma and relapsed anaplastic large cell lymphoma (Nat. Biotechnol. (2003) 21(7):778-784; WO2004010957; WO2005001038; U.S. Pat. Nos. 7,090,843A; 7,659,241; WO2008025020). Adcetris® is a novel targeting ADC drug, which causes the drug to act directly on the target CD30 of lymphoma cells, trigger endocytosis and consequently induce tumor cell apoptosis.
In February 2013, Kadcyla® (ado-trastuzumab emtansine, T-DM1) gained approval from the FDA for the treatment of advanced or metastatic breast cancer patients who are HER2-positive with trastuzumab- (trade name: Herceptin®) and paclitaxel-resistant (WO2005037992; U.S. Pat. No. 8,088,387). Both Mylotarg® and Adcetris® are target therapies for hematologic tumors, the organizational structure of which is relatively simple compared to that of solid tumors. Kadcyla® is the first ADC drug approved by the FDA for the treatment of solid tumors.
Kadcyla®, which uses ImmunoGen technology, is formed by conjugating a highly active mitosis inhibitor DM1 and Roche's trastuzumab via a stable thioether bond linker (T-DM1), wherein the average drug load of one trastuzumab is about 3.5 per DM1. Trastuzumab specifically binds to breast cancer cells in a patient, and is cleaved to release the DM1 intracellularly after endocytosis. The intracellular aggregation concentration of DM1 is sufficient to cause cell death due to mitotic disturbance, followed by regression of tumor focus (unlike Herceptin® mAb monotherapy, which often results in retardation of tumor growth). T-DM1 not only retains antibody-dependent inhibition of cell proliferation like Herceptin®, but it also increases the potential effect of cytotoxic drug. Also, because its toxins are released in the target tumor cells, the side effects thereof are not simultaneously increased with its increasing curative effect.
Pertuzumab (also known as 2C4, trade name of Perjeta) is a recombinant humanized monoclonal antibody, which was first called “HER dimerization inhibitor”. Pertuzumab blocks the dimerization of HER2 and other HER receptors by binding to HER2 (Agus D B, (2002) Cancer Cell (2):127-137; Schaefer G, (1997) Oncogene (15): 1385-1394; Mendoza N, (2002) Cancer Res (62): 5485-5488; Takai N, (2005) Cancer (104): 2701-2708; Jackson J G, (2004) Cancer Res (64): 2601-2609). It has been verified that pertuzumab has an inhibitory effect on tumor growth in both HER2-high-expression and low-expression prostate cancer models (Craft N, (1999) Nat Med (5):280-285; Oxley J D, (2002) J Clin Pathol (55): 118-120; Reese D M, (2001) Am J Clin Pathol (116): 234-239; Agus D B, (2002) Cancer Cell (2): 127-137).
Being different from Trastuzumab (trade name Herceptin), which inhibits the downstream signaling pathways via the binding site located on the juxtamembrane region IV sub-domain of the HER2 extracellular domain, pertuzumab effectively inhibits the heterologous dimerization of HER2 via binding to domain II (dimerization domain). Therefore, trastuzumab only has some effect on patients with HER2 over-expressed cancer, especially on breast cancer patients. Although sharing the same target and endocytosis with trastuzumab, due to its different mechanism of action, pertuzumab can block the signaling pathway mediated by ErbB family receptor after inhibiting dimerization, and may have a more extensive application than the one only blocking the HER2 signaling pathway (Franklin M C, (2001) Cancer Cell (5): 317-328).
Currently, there are mainly two techniques for conjugation of ADC drug: for T-DM1, random conjugation of cytotoxic drug and free amino groups in the antibody is used (WO2005037992); while for Adcetris®, conjugation of cytotoxic drug and free thiol groups in the antibody after hinge region reduction (WO2004010957) is used. Both conjugation methods produce a mixture with an inconsistent Drug to Antibody Ratio. For example, the average Drug to Antibody Ratio of T-DM1 is 3.5, however, the drug loading distribution is from 0 to 8. A Low Drug to Antibody Ratio affects ADC efficacy, while a high Drug to Antibody Ratio more easily leads to excessive antibody modification, resulting in ADC drug recognition and destruction by the tissue macrophage system. This not only shortens the half-life of the ADC, but also increases the toxic side effects on account of accumulation of toxins in non-target tissues; and for Adcetris®, the disulfide bond of the antibody hinge region is reduced with a reducing agent, which would have a certain impact on the stability of the antibody itself.